The present disclosure relates generally to ink jet printers, and more particularly, to detection of ink jet operation within ink jet printers.
Ink-jet printing systems commonly utilize either direct printing or offset printing architectures. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving medium. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving medium is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the medium.
In some direct and offset printing systems, the print head moves relative to the final receiving medium or the intermediate transfer surface in two directions as the print head jets are fired. Typically, the print head is translated at least somewhat along the X-axis, or cross-process direction, while the final receiving medium or intermediate transfer surface is moved along the Y-axis or process direction. In this manner, the print head effectively “scans” over the print medium and forms a dot-matrix image by selectively depositing ink drops at specific locations on the medium.
In the offset printing architecture, the print head typically moves in the X-axis direction that is parallel to the axis of the rotating drum on which the intermediate transfer surface is supported. Typically, the print head includes multiple jets configured in one or more linear arrays that print a set of scan lines on the intermediate transfer surface with each drum rotation. Precise placement of the scan lines is necessary to meet image resolution requirements and to avoid producing undesired printing artifacts, such as banding or streaking. Accordingly, the X-axis (head translation) and the Y-axis (drum rotation) motions must be carefully coordinated with the firing of the jets to insure proper scan line placement.
Offset printers often employ print heads that have jets extending over a significant width (X-axis displacement) in order to increase printing speed. In general, a wider print head requires less X-axis movement and can reduce the number of revolutions required to print an image. For example, if a print head extends the entire axial length of the drum and contains as many ink jets as are necessary to print the required resolution, then printing may be achieved by simply rotating the drum a single time and no head translation would be required. Alternatively, multiple print heads that are arranged in such manner as to combine to extend over the entire X-axis direction may also achieve single revolution or “single pass” printing. Such printers are known.
Still other designs employ at least some X-axis displacement in order to reduce the number of ink jets required to achieve a particular resolution. For example, if an x-axis resolution of 600 dots per inch (DPI) is desired in a printer having a usable printing width of 12 inches, then a single pass printer (with no X-axis displacement) would require 7200 ink jets per ink color. However, a multipass printer having twenty-four passes using some degree of X-axis displacement would only require 300 ink jets per ink color to achieve the same resolution image. Specifically, the three hundred ink jets would move to discrete, unique X-axis positions on each of the twenty-four printing passes, so that all 7200 pixels are ultimately covered.
Multipass printers typically employ interleaving or interlacing to achieve the desired resolution. In interleaved or interlaced printing, the print head includes jets that are spaced apart by a distance greater than the final DPI resolution. During each pass of the intermediate transfer surface, the jets are aligned to print at select, spaced-apart X-axis positions. Accordingly, each pass only prints a fraction of the total X-axis position within the overall width of the print head. After each pass, the print head translates in the X-axis direction by a distance that aligns the jets to print along previously unprinted X-axis positions. Eventually, after a certain number of passes, all of the pixel columns that fall within the width of the printable space have been exposed to a jet.
Similar to single pass printers, multi-pass printers may employ multiple staggered print heads to further improve the printing speed. Staggered print heads may effectively extend over large portions of the X-axis dimension to reduce the amount of X-axis scanning displacement while employing smaller and less expensive print head hardware.
In all of the above designs, as well as others, it is important to ensure the accuracy of X-axis and Y-axis alignment of the jets during the printing process.
A method disclosed herein includes ejecting ink from a first set of one or more nozzles to form a first image pattern at least partially within an image field. The method likewise includes ejecting ink from a second set of one or more nozzles to form a second image pattern at least partially within the image field. The first image pattern and second image pattern form a composite pattern within the image field. Then, an optical detector obtains a measurement of an optical characteristic of the image field. The measurement may be compared with a reference optical characteristic for the composite pattern within the image field.
The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.